A data management system for storing user data into a management side's large capacity memory via a network is known.
In case of conventional system where a custodian holds a user's “article”, when the user asks the custodian to dispose of the article, the custodian simply discards the article itself. However, in the data management system as above, since data can be easily duplicated, even if data is deleted, an unauthorized duplication of the data might be stored in the memory. It is very difficult for the management side to prove that the data has been deleted, and it is also very difficult for the user to verify the deletion of the data.